Ceramic precursor batch composition and method of increasing ceramic precursor batch extrusion rate

ABSTRACT

A ceramic precursor batch composition comprising inorganic ceramic-forming ingredients, a hydrophobically modified cellulose ether binder having a molecular weight less than or equal to about 300,000 g/mole and an aqueous solvent is provided. The ceramic precursor batch composition has a ratio of binder to aqueous solvent of less than about 0.32. The ceramic precursor batch composition may be used to increase the rate of extrusion of the composition. A method for increasing a rate of extrusion of a ceramic precursor batch composition is also disclosed.

RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.12/275,007 filed on Nov. 20, 2008, which claims priority to and thebenefit of U.S. Provisional Patent Application No. 61/004,996 filed onNov. 30, 2007, both of which are relied upon and hereby incorporated byreference for all purposes as if fully set forth herein.

FIELD

The present invention relates generally to ceramic precursor batchcompositions, and particularly to ceramic precursor batch compositionsfor use in extruding ceramic honeycombs.

BACKGROUND

Hydrophobically modified cellulose polymers such as methylcellulose (MC)and hydroxypropyl methylcellulose (HPMC) have been used as binders inautomotive substrate and diesel filter ceramic precursor batchcompositions. These polymers give the batch the necessary plasticity andgreen strength in the forming and drying stages to produce high qualityhoneycomb ware. However, polymers such as MC and HPMC can undergo phaseseparation and subsequent gelation at a characteristic temperature. Atthe right temperature the polymer loses the water that surrounds thependant methoxy side groups. This loss of hydration exposes the methoxygroups and enables hydrophobic associations to occur between the methoxysubstituents of neighboring chains. This leads to phase separation andultimately the build up of a long range network gel. (Sarkar, N., J.Appl. Polym. Sci, 24, 1073-1087 (1979); Methocel Cellulose EthersTechnical Handbook, Dow Chemical Co.; Li, L et al., Langmuir, 18,7291-7298 (2002)). When the binder undergoes this thermal phasetransition within a ceramic precursor batch, the batch becomes stifferand the extrusion pressure increases significantly which can producesevere defects in the extruded honeycomb structure.

The thermal transition behavior of polymers like MC and HPMC can limitthe extrusion process of numerous ceramic product lines. For example,production will have to significantly increase the extrusion feedrate ofnew diesel compositions such as aluminum titanate (AT) and advancedcordierite (AC) in the next 1-2 years due to increased demand for dieselproducts. However, the batch temperature increases with feedrate due toincreased shear heating in the extruder. Ultimately, throughput reachesa limit as the batch approaches the thermal transition temperature ofthe binder.

SUMMARY

Accordingly, in light of the desire to increase federate, it would bedesirable to have a ceramic precursor batch composition that allows fora greater extrusion feedrate. Such a ceramic precursor batch may stiffenat higher temperatures without sacrificing the properties of the finalproduct such as, but not limited to, strength.

One aspect of the invention is a ceramic precursor batch compositioncomprising inorganic ceramic-forming ingredients, a hydrophobicallymodified cellulose ether binder having a molecular weight less than orequal to about 300,000 g/mole and an aqueous solvent, wherein MC/W isless than about 0.32, MC is a weight % of the hydrophobically modifiedcellulose ether binder based on a 100% of the inorganic ceramic-formingingredients, and W is a weight % of aqueous solvent based on the 100% ofthe inorganic ceramic-forming ingredients. There is an inverse linearrelationship between MC/W and the stiffening onset temperature. KeepingMC/W less than about 0.32, or even less than 0.22, allows for increasedfederates for the composition of the present invention.

In another aspect, the present invention includes a method forincreasing a rate of extrusion of a ceramic precursor batch composition,comprising the steps of providing inorganic ceramic-forming ingredients,adding a hydrophobically modified cellulose ether binder and water tothe inorganic ceramic forming ingredients wherein the hydrophobicallymodified cellulose ether binder has a molecular weight of less thanabout 300,000 g/mole and MC/W is less than about 0.32, wherein MC is aweight % of the hydrophobically modified cellulose ether binder based ona 100% of the inorganic ceramic-forming ingredients, and W is a weight %of the water based on the 100% of the inorganic ceramic-formingingredients.

In yet another aspect, the present invention includes a method forincreasing a rate of extrusion of a ceramic precursor batch composition,comprising the steps of providing an initial ceramic precursor batchcomposition including inorganic ceramic-forming ingredients, a highmolecular weight hydrophobically modified cellulose ether binder havinga molecular weight of greater than about 300,000 g/mole, and water,substituting a low molecular weight hydrophobically modified celluloseether binder having a molecular weight of less than about 300,000 g/molefor the high molecular weight binder, and adjusting a ratio of MC/W tobe less than about 0.32 wherein MC is a weight % of the hydrophobicallymodified cellulose ether binder based on a 100% of the inorganicceramic-forming ingredients, and W is a weight % of the water based onthe 100% of the inorganic ceramic-forming ingredients.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate various embodiments of theinvention, and together with the description serve to explain theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical illustration showing the relationship betweenbinder molecular weight and the stiffening onset temperature, accordingto one embodiment of the present invention;

FIG. 2 is a graphical illustration showing the relationship between MC/Wand the stiffening onset temperature, according to another embodiment ofthe present invention;

FIG. 3 is a graphical illustration showing the relationship between MC/Wand the stiffening onset temperature for different batch compositions,according to one embodiment of the present invention;

FIG. 4 is a graphical illustration showing the relationship between MC/Wand the stiffening onset temperature for different batch compositions,according to one embodiment of the invention; and

FIG. 5 is a graphical illustration showing an example of a capillarytemperature sweep of an AT batch, according to the present invention.

DETAILED DESCRIPTION

Broadly, the present invention provides a ceramic precursor batchcomposition with a higher stiffening onset temperature, allowing forgreater extrusion federates without significant increases in pressure.The composition may comprise inorganic ceramic-forming ingredients, ahydrophobically modified cellulose ether binder having a molecularweight less than or equal to about 300,000 g/mole and an aqueous solventsuch as, but not limited to, water. The ceramic precursor batchcomposition may have a MC/W less than about 0.32 where MC is a weight %of the hydrophobically modified cellulose ether binder based on a 100%of the inorganic ceramic-forming ingredients, and W is a weight % ofwater based on the 100% of the inorganic ceramic-forming ingredients.There is also provided a method for increasing a rate of extrusion(feedrate) of a ceramic precursor batch composition, comprising usingthe ceramic precursor batch composition of the present invention.

The ceramic precursor batch composition of the present invention useslower molecular weight hydrophobically modified cellulose ether bindersand a lower MC/W ratio to provide a batch composition that has a higherstiffening onset temperature, a lower pressure during extrusion and agreater feedrate than the ceramic precursor batch compositions of theprior art using higher molecular weight hydrophobically modifiedcellulose ethers. It has been found that the MC/W ratio is inverselyproportional to the stiffening onset temperature of the ceramicprecursor batch composition. Additionally, it has also been found thatlower molecular weight hydrophobically modified cellulose ether bindersare more effectively hydrated, allowing for a lower MC/W ratio, whichresults in a higher stiffening onset temperature. Therefore, a lowmolecular weight binder may be substituted in a composition with allother parameters being equal to obtain a higher stiffening onsettemperature. In contrast, the prior art attempts to solve the problem oflower stiffening temperatures and lower feedrates by either increasingthe MC/W ratio, which can result in a weaker green body, or by includingadditional ingredients. The present invention does not rely onadditional ingredients or increased solvent amounts.

In accordance with the invention, the present invention for a ceramicprecursor batch composition includes inorganic ceramic-formingingredients, a hydrophobically modified cellulose ether binder having amolecular weight less than or equal to about 300,000 g/mole and anaqueous solvent. The MC/W ratio is less than about 0.32 where MC is aweight % of the hydrophobically modified cellulose ether binder based ona 100% of the inorganic ceramic-forming ingredients, and W is a weight %of aqueous solvent based on the 100% of the inorganic ceramic-formingingredients. It will be appreciated that the weight percents of thebinder, solvent, and other additives are calculated as superadditionswith respect to the inorganic ceramic-forming ingredients by thefollowing formula:

$\frac{{{weight}\mspace{14mu} {of}\mspace{14mu} {binder}},{{solvent}\mspace{14mu} {or}\mspace{14mu} {other}\mspace{14mu} {additives}}}{\begin{matrix}{{{weight}\mspace{14mu} {of}\mspace{14mu} {inorganic}\mspace{14mu} {ceramic}} -} \\{{{forming}\mspace{14mu} {ingredients}} + {poreformer}}\end{matrix}} \times 100$

The inorganic ceramic-forming ingredients may be cordierite, mullite,clay, talc, zircon, zirconia, spinel, aluminas and their precursors,silicas and their precursors, silicates, aluminates, lithiumaluminosilicates, feldspar, titania, fused silica, nitrides, carbides,borides, e.g., silicon carbide, silicon nitride, soda lime,aluminosilicate, borosilicate, soda barium borosilicate or combinationsof these, as well as others. Combinations of these materials may bephysical or chemical combinations, for example, mixtures or composites,respectively.

In one exemplary embodiment, the inorganic ceramic-forming ingredientsmay yield an aluminum-titanate ceramic material upon firing. In anotherexemplary embodiment, the inorganic ceramic-forming ingredients may bethose that yield cordierite, mullite, or mixtures of these on firing,some examples of such mixtures being about 2% to about 60% mullite, andabout 30% to about 97% cordierite, with allowance for other phases,typically up to about 10% by weight. Some ceramic batch materialcompositions for forming cordierite that are especially suited to thepractice of the present invention are those disclosed in U.S. Pat. No.3,885,977 which is herein incorporated by reference as filed.

One composition, by way of a non-limiting example, which ultimatelyforms cordierite upon firing is as follows in percent by weight,although it is to be understood that the invention is not limited tosuch: about 33-41, and most preferably about 34-40 of aluminum oxide,about 46-53 and most preferably about 48-52 of silica, and about 11-17and most preferably about 12-16 magnesium oxide.

In the practice of the present invention, the ceramic precursor batchcomposition comprising the binder system and an inorganic powdercomponent consisting of a sinterable inorganic particulate material,e.g., a ceramic powder material, can be prepared by using the componentsin any desired amounts selected.

The inorganic ceramic-forming ingredients can be synthetically producedmaterials such as oxides, hydroxides, etc., or they can be naturallyoccurring minerals such as clays, talcs, or any combination of these.The invention is not limited to the types of powders or raw materials.These can be chosen depending on the properties desired in the body.

The hydrophobically modified cellulose ether binder may be, but notlimited to, methylcellulose, ethylhydroxy ethylcellulose, hydroxybutylmethylcellulose, hydroxymethylcellulose, hydroxypropyl methylcellulose,hydroxyethyl methylcellulose, hydroxybutylcellulose,hydroxyethylcellulose, hydroxypropylcellulose, sodium carboxymethylcellulose, and mixtures thereof. Methylcellulose and/ormethylcellulose derivatives are especially suited as organic binders inthe practice of the present invention with methylcellulose,hydroxypropyl methylcellulose, or combinations of these being preferred.Preferred sources of cellulose ethers are METHOCEL A4M, F4M, F240, andK75M cellulose products from Dow Chemical Co. METHOCEL A4M cellulose isa methylcellulose. METHOCEL F4M, F240, and K75M cellulose products arehydroxypropyl methylcellulose.

The properties of preferred cellulose ether binders such asmethylcellulose are water retention, water solubility, surface activityor wetting ability, thickening of the mixture, dispersing andlubricating of the inorganic particles, providing wet and dry greenstrength to the green bodies, thermal gelation and hydrophobicassociation in an aqueous environment. Cellulose ether binders thatpromote hydrogen bonding interaction with the solvent are desirable.Examples of substituent groups that maximize the hydrogen bondinginteraction with polar solvents e.g. water, are hydroxypropyl andhydroxyethyl groups, and to a smaller extent hydroxybutyl groups. Whilenot wishing to be bound by theory, it is believed that lower molecularweight cellulose ether binders, i.e. less than or equal to about 300,000g/mole, hydrate faster than cellulose ether binders having a molecularweight greater than 300,000 g/mole. The faster hydration of the lowermolecular weight cellulose ether binders requires less water, allowingfor a stiffer and stronger composition.

The hydrophobically modified cellulose ether binders may have amolecular weight of less than of equal to about 300,000 g/mole. In oneexemplary embodiment the cellulose ether binder has a molecular weightof from about 50,000 g/mole to about 300,000 g/mole. In anotherexemplary embodiment, the cellulose ether binder has a molecular weightof from about 100,000 g/mole to about 200,000 g/mole. In a furtherexemplary embodiment the cellulose ether binder has a molecular weightof from about 100,000 g/mole or from about 200,000 g/mole. By way ofcomparison, commonly used METHOCEL F220M and METHOCEL F40M (F240) havemolecular weights of 374,450 g/mole and 309,500 g/mole respectivelywhile METHOCELS F4M and F50 have molecular weights of 178,850 g/mole and75,650 g/mole respectively.

As illustrated in FIG. 1, the stiffening onset temperature (T_(onset))is inversely proportional to the molecular weight of the cellulose etherbinder. Historically the term batch “gelation temperature” has been usedto define the point at which the binder undergoes its thermal phasetransition. However, since polymers such as methylcellulose andhydroxypropyl methylcellulose phase separate and then subsequently gel,it's not clear which phenomenon causes the batch viscosity to increase.Therefore, to avoid confusion the more general term of “onsettemperature” (T_(onset)) will be used herein to describe the temperatureat which the batch viscosity, or stiffness, begins to increase. METHOCELF220M, with the greatest molecular weight has the lowest T_(onset) whileF50, with the lowest molecular weight has the highest T_(onset). It willbe appreciated that the higher the T_(onset), the less likely stiffeningwill occur during extrusion, causing increased pressures and decreasedfeedrates.

The hydrophobically modified cellulose ether binder may be a combinationof at least two different hydrophobically modified cellulose ethers,where the average molecular weight of the combination is less than orequal to about 300,000 g/mole. Combining a higher molecular weightMETHOCEL, such as F40M, with a lower molecular weight cellulose ethermay give a green body with increased strength over using only a lowmolecular weight cellulose ether such as METHOCEL F50. The combinationof cellulose ethers may comprise cellulose ethers having differentmolecular weights. Alternatively, the combination of cellulose ethersmay comprise cellulose ethers having different hydrophobic groups,different concentrations of the same hydrophobic group or othercombinations. Different hydrophobic groups may be, by way ofnon-limiting example, hydroxyethyl or hydroxypropyl.

The hydrophobically modified cellulose ether binder makes up, as asuperaddition, typically about 1-10% by weight, and more typically about2-6% by weight of the inorganic ceramic-forming material.

The solvent provides a medium for the binder to dissolve in thusproviding plasticity to the batch and wetting of the powders. Thesolvent can be aqueous based, which are normally water or water-misciblesolvents; or organically based. Most useful are aqueous based solventswhich provide hydration of the binder and powder particles. Typically,the amount of aqueous solvent is from about 20% by weight to about 50%by weight.

It was unexpectedly found that there is a strong correlation between theratio of cellulose ether binder to aqueous solvent (MC/W) and T_(onset)wherein T_(onset) decreases linearly with increasing cellulose etherbinder concentrations (FIGS. 2 and 3). In contrast, there wascorrelation of T_(onset) with the amount of water present. FIG. 2illustrates the relationship between MC/W using METHOCEL F40M in waterand aluminum-titanate ceramic-forming materials. There is about a 20° C.difference between the highest MC/W ratio of about 0.34 to the lowest ofabout 0.20. FIG. 3 illustrates the relationship between MC/W usingMETHOCEL F40M in water and different inorganic ceramic forming materialswhere CA-CF are cordierite compositions comprising silica, alumina,magnesia made from batches including clay, talc, silica and an aluminaforming source and an optional pore former, and AT is aluminum titanateceramic forming material. The T_(onset) decreases with increasingMETHOCEL concentration across all seven ceramic compositions tested.This result indicates that T_(onset) is primarily driven by the aqueousMETHOCEL concentration and that there is little influence from otherbatch parameters such as the chemistry of the inorganic components,particle size distribution, presence of oil lubricants, or surfactanttype such as tall oil, stearic acid, or Liga (sodium stearate). Thefundamental aspect that determines T_(onset) appears to be the degree towhich the methylcellulose ether or hydroxypropyl methylcellulose etherpolymer chains are hydrated.

The correlation between MC/W and T_(onset) is observed with otherMETHOCEL binders having lower molecular weights than F40M, asillustrated in FIG. 4. Three METHOCELS of different molecular weightswere used, F40M (M1), F4M (M2) and F50 (M3). Different ceramic precursormaterials were also used, such as aluminum-titanate (AT) and cordierite(Cord). For all METHOCEL binders, there is a 10-20° C. increase inT_(onset) when MC/W is reduced.

In one exemplary embodiment, the MC/W ratio of the ceramic precursorbatch composition of the present invention is less than 0.32. In anotherexemplary embodiment, the MC/W is less than 0.27. In a further exemplaryembodiment, the MC/W is less than 0.22.

Decreasing MC/W results in increasing the amount of aqueous solventpresent. It will be appreciated that increasing the amount of aqueoussolvent present may increase the drying time of the green bodies formedfrom a composition with a higher MC/W. However, using a lower molecularweight cellulose ether binder can result in an increased T_(onset)without significantly lowering the MC/W ratio.

The ceramic precursor batch composition of the present invention mayfurther comprise other additives such as surfactants, oil lubricants andpore-forming material. Non-limiting examples of surfactants that can beused in the practice of the present invention are C₈ to C₂₂ fatty acidsand/or their derivatives. Additional surfactant components that can beused with these fatty acids are C₈ to C₂₂ fatty esters, C₈ to C₂₂ fattyalcohols, and combinations of these. Exemplary surfactants are stearic,lauric, oleic, linoleic, palmitoleic acids, and their derivatives,stearic acid in combination with ammonium lauryl sulfate, andcombinations of all of these. Most preferred surfactants are lauricacid, stearic acid, oleic acid, and combinations of these. The amount ofsurfactants typically may be from about 0.5% by weight to about 2% byweight.

Non-limiting examples of oil lubricants are light mineral oil, corn oil,high molecular weight polybutenes, polyol esters, a blend of lightmineral oil and wax emulsion, a blend of paraffin wax in corn oil, andcombinations of these. Typically, the amount of oil lubricants may befrom about 1% by weight to about 10% by weight. In an exemplaryembodiment, the oil lubricants are present from about 3% by weight toabout 6% by weight.

In filter applications, such as in diesel particulate filters, it iscustomary to include a burnout poreformer in the mixture in an amounteffective to subsequently obtain the porosity required for efficientfiltering. A burnout poreformer is any particulate substance (not abinder) that burns out of the green body in the firing step. Some typesof burnout agents that can be used, although it is to be understood thatthe invention is not limited to these, are non-waxy organics that aresolid at room temperature, elemental carbon, and combinations of these.Some examples are graphite, cellulose, flour, etc. Elemental particulatecarbon is preferred. Graphite is especially preferred because it has theleast adverse effect on the processing. In an extrusion process, forexample, the rheology of the mixture is good when graphite is used.Typically, the amount of graphite is about 10% to about 30%, and moretypically about 15% to about 30% by weight based on the inorganicmaterial. If a combination of graphite and flour are used, the amount ofburnout agent is typically form about 10% by weight to about 25% byweight with the graphite at 5% by weight to 10% of each and the flour at5% by weight to about 10% by weight.

The present invention also provides a method for increasing a rate ofextrusion of a ceramic precursor batch composition, comprising the stepsof providing inorganic ceramic-forming ingredients, adding ahydrophobically modified cellulose ether binder and water to theinorganic ceramic forming ingredients wherein the hydrophobicallymodified cellulose ether binder has a molecular weight of less thanabout 300,000 g/mole and MC/W is less than about 0.32, wherein MC is aweight % of the hydrophobically modified cellulose ether binder based ona 100% of the inorganic ceramic-forming ingredients, and W is a weight %of the water based on the 100% of the inorganic ceramic-formingingredients. The inorganic materials, binder and water are mixed in amuller or plow blade mixer. The water is added in an amount that is lessthan is needed to plasticize the batch. With water as the solvent, thewater hydrates the binder and the powder particles. The surfactantand/or oil lubricant, if desired, may then be added to the mix to wetout the binder and powder particles.

The composition is then plasticized by shearing the wet mix formed abovein any suitable mixer in which the batch will be plasticized, such asfor example in a twin-screw extruder/mixer, auger mixer, muller mixer,or double arm, etc. Extent of plasticization is dependent on theconcentration of the components (binder, solvent, surfactant, oillubricant and the inorganics), temperature of the components, the amountof work put in to the batch, the shear rate, and extrusion velocity.During plasticization, the binder dissolves in the solvent and a gel isformed. The gel that is formed is stiff because the system is verysolvent-deficient. The surfactant enables the binder-gel to adhere tothe powder particles.

In a further step, the composition is extruded to form green bodies.Extrusion is done with devices that provide low to moderate shear. Forexample hydraulic ram extrusion press, which is the preferred device, ortwo stage de-airing single auger are low shear devices. A single screwextruder is a moderate shear device. The extrusion can be vertical orhorizontal. Another example of forming the green bodies is using thesame plasticizing twin-screw extruder as the forming extruder whenappropriate forming dies are used, as a single step process.

The bodies of this invention can have any convenient size and shape andthe invention is applicable to all processes in which plastic powdermixtures are shaped. The process is especially suited to production ofcellular monolith bodies such as honeycombs. Cellular bodies find use ina number of applications such as catalytic, adsorption, electricallyheated catalysts, filters such as diesel particulate filters, moltenmetal filters, regenerator cores, etc.

Generally honeycomb densities range from about 235 cells/cm.sup.2 (1500cells/in.sup.2) to about 15 cells/cm.sup.2 (100 cells/in.sup.2).Examples of honeycombs produced by the process of the present invention,although it is to be understood that the invention is not limited tosuch, are those having about 94 cells/cm.sup.2 (about 600cells/in.sup.2), or about 62 cells/cm.sup.2 (about 400 cells/in.sup.2)each having wall thicknesses of about 0.1 mm (4 mils). Typical wallthicknesses are from about 0.07 to about 0.6 mm (about 3 to about 25mils), although thicknesses of about 0.02-0.048 mm (1-2 mils) arepossible with better equipment. The method is especially suited forextruding thin wall/high cell density honeycombs.

The extrudates can then be dried and fired according to knowntechniques. The firing conditions of temperature and time depend on thecomposition and size and geometry of the body, and the invention is notlimited to specific firing temperatures and times. For example, incompositions which are primarily for forming cordierite, thetemperatures are typically from about 1300° C. to about 1450° C., andthe holding times at these temperatures are from about 1 hour to about 6hours. For mixtures that are primarily for forming mullite, thetemperatures are from about 1400° C. to about 1600° C., and the holdingtimes at these temperatures are from about 1 hour to about 6 hours. Forcordierite-mullite forming mixtures which yield the previously describedcordierite-mullite compositions, the temperatures are from about 1375°C. to about 1425° C. Firing times depend on factors such as kinds andamounts of materials and nature of equipment but typical total firingtimes are from about 20 hours to about 80 hours.

EXAMPLES

The invention will be further clarified by the following examples.

Example 1

Batch stiffening onset temperature (T_(one)). Historically the termbatch “gelation temperature” has been used to define the point at whichthe binder undergoes its thermal phase transition. However, sincepolymers such as methylcellulose and hydroxypropyl methylcellulose phaseseparate and then subsequently gel, it's not clear which phenomenoncauses the batch viscosity to increase. Therefore, to avoid confusion wewill use the more general term of “onset temperature” (T_(onset)) todescribe the temperature at which the batch viscosity, or stiffness,begins to increase.

The batch stiffening temperature of all batch samples was measured byusing the capillary temperature sweep method. A Malvern RH7 capillaryrheometer was used to extrude batch through two OEM capillary dies madeof tungsten carbide. One die has an L/d of 16 (1 mm diameter) while theother is an orifice die with an L/d of 0.25. The batch is extruded at alinear extrudate velocity of 12.7 mm/s at a temperature ramp rate of 1°C./min. FIG. 5 shows data from a typical temperature sweep test of AT.

Only the data from the orifice die is used to determine T_(onset)because this die almost always produces a flat baseline pressure duringthe temperature ramp. This baseline pressure is essential for howT_(onset) is defined. Starting at five degrees above the initialtemperature of the scan, the pressure over the next 15 degrees wasaveraged. The average pressure over this 15° C. window is termedP_(avg). Pressure data from the first five degrees was not used in orderto avoid any pressure start up effects which could alter P_(avg). Thefifteen degree window provides a sufficient amount of data to establisha baseline pressure. Once P_(avg) was obtained, a pressure that is 15%higher than this value was calculated. T_(onset) is taken to be thetemperature which is at a value 1.15 P_(avg). On an extruder that isoperating near the binder gel point, an increase in extrusion pressureof 15% above a stable pressure is indicative of a significant change inbatch rheology related to the binder transition.

The capillary temperature sweep method used to measure T_(onset) hasseveral advantages over the conventional method of using a Brabendermixer. First, the orifice die pressure has much less noise andfluctuation than typical torque readings produced by a Brabender.Second, the capillary method measures T_(onset) of a batch sample in afixed shear history state. This can not be done with the Brabendermethod because the Brabender shears the batch during a temperature rampand therefore the level of specific mixing energy imparted to thematerial increases continuously during the test.

Example 2

Effect of hydroxypropyl methylcellulose concentration on T_(onset). Areview of the literature shows that the findings are mixed with regardto how the gel point of methyl cellulose and hydroxypropylmethylcellulose solutions are affected by polymer concentration. Somereports indicate that the gel temperature of hydroxypropylmethylcellulose solutions decreases linearly with increasingconcentration up to 10 wt %. Unfortunately these results have not beenreproduced, even after significant efforts to do so. In fact, thedynamic thermorheological results agree with those reported in the priorart who saw no dependence of polymer concentration on the gel points ofE, F, and K-type HPMC solutions.

Though there have been numerous studies on the gelation of methylcellulose and hydroxypropyl methylcellulose solutions, there has beenvery little reported on how the concentration of methyl cellulose andhydroxypropyl methylcellulose affects the stiffening temperature of ahighly filled ceramic batch.

A study with AT was conducted to determine how binder concentrationaffects T_(onset) in an actual ceramic batch composition used inproduction. AT samples were prepared by twin screw machine in fourseparate investigations. Two sets of tests were conducted in the labwhere we measured batch stiffening temperature T_(onset) of 13 mmdiameter rods extruded directly from a 34 mm twin screw machine. Theother two sets of tests were conducted on samples prepared on a 90 mmproduction twin screw machine. The honeycomb ware samples from the 90 mmextruder were compressed into blocks using a hydraulic press. 13 mmdiameter rods were cored from these blocks to load into the capillaryrheometer for temperature ramp testing.

Using the 90 mm extruder, F40M METHOCEL binder (Dow Chem. Co.) was usedin all samples and measured T_(onset) at 3.5, 4.0 and 4.5% METHOCEL andtwo water calls at each METHOCEL level. As the level of METHOCEL wasreduced, it was observed that the batch began to stiffen at a highertemperature.

T_(onset) clearly showed a linear relationship with METHOCEL to waterratio in the AT composition. However, production samples of AT typicallyhave a T_(onset) value in the low 30's ° C. which is much lower thancompositions of cordierite. T_(onset) of CA was approximately 20° C.higher than that of AT even though both compositions use the exact samebinder. The only time such a large difference in stiffening temperaturehas been observed is when two different binder chemistries have beencompared such as an A-type methycellulose versus a K-type hydroxypropylmethylcellulose. It is hypothesized that the large differences inT_(onset) could be due to the differences in METHOCEL concentration.

Temperature sweep data for a variety of auto and diesel compositionswere used to determine if binder concentration was responsible for thedifferences in T_(onset). T_(onset) values for seven ceramic auto anddiesel compositions were examined. All samples contained F40M METHOCELand were made with either the 34 mm twin screw, 90 mm production twinscrew, or were plasticized in a Brabender mixer for 10-20 minutes tosimulate the plasticizing step that occurs in a twin screw machine. FIG.3 shows a plot of T_(onset) for all ceramic compositions versus theMETHOCEL/water ratio.

The results in FIG. 3 show that there is a strong linear correlationbetween T_(onset) and the aqueous concentration of binder in the batch.The batch stiffening temperature decreases with increasing METHOCELconcentration across all seven ceramic compositions tested. This resultindicates that T_(onset) is primarily driven by the aqueous METHOCELconcentration and that there is little influence from other batchparameters such as the chemistry of the inorganic components, particlesize distribution, presence of oil lubricants, or surfactant type suchas tall oil, stearic acid, or Liga (sodium stearate). The fundamentalaspect that determines T_(onset) appears to be the degree to which themethyl cellulose and hydroxypropyl methylcellulose polymer chains arehydrated.

Example 3

Effect of binder molecular weight on T_(onset). Another secondaryparameter that can impact T_(onset) is the molecular weight of thebinder. There is one report in the prior art that shows the impact ofthe binder molecular weight on the gelation temperature of a ceramicbatch. (Scheutz, J. E. Ceramic Bulletin, 65, 1556-1559 (1986). In thisreport a Brabender mixer was used to measure the torque as a function oftemperature of alumina batch samples using K4M and K15M viscosity gradeMETHOCELS as the binders at 2.5% and 5% loading. The results of the 2.5%binder test showed that the lower viscosity (i.e. lower molecularweight) K4M binder had a gelation temperature 14° C. above that of thehigher molecular weight K15M binder. At 5% binder level, the K4M had agelation temperature 8° C. higher than K15M. Since only two molecularweights were used in this study, there is no way to determine if thereis a well-defined relationship between binder molecular weight andgelation temperature. In addition to this, the batch gelationtemperatures were not compared on equal mixing energy basis since mixingenergy was not controlled.

The effect molecular weight has on T_(onset) of an AT batch was measuredin a controlled experiment using four viscosity grades of an F-typeMETHOCEL: F220M, F40M, F4M, and F50 with F220M being the highestmolecular weight and F50 being the lowest. A 4.5% binder and 16% EmulsiaT plus 2% additional water in each sample were used. All samples wereplasticized in the muller prior to extruding rods for the capillarytemperature sweep.

The results showed that the onset temperature increased with decreasingbinder molecular weight. T_(onset) has a very well-correlated dependenceon molecular weight as is shown in FIG. 1.

What is claimed is:
 1. A ceramic precursor batch composition,comprising: inorganic ceramic-forming ingredients; a hydrophobicallymodified cellulose ether binder having a molecular weight less than orequal to about 300,000 g/mole; an aqueous solvent; and wherein MC/W isless than about 0.32, MC is a weight % of the hydrophobically modifiedcellulose ether binder based on a 100% of the inorganic ceramic-formingingredients, and W is a weight % of water based on the 100% of theinorganic ceramic-forming ingredients.
 2. The ceramic paste compositionof claim 1 wherein the hydrophobically modified cellulose ether binderhas a molecular weight of from about 50,000 g/mole to about 300,000g/mole.
 3. The ceramic paste composition of claim 1 wherein thehydrophobically modified cellulose ether binder has a molecular weightof from about 100,000 g/mole to about 200,000 g/mole.
 4. The ceramicprecursor batch composition of claim 1 wherein the hydrophobicallymodified cellulose ether binder has a molecular weight of from about200,000 g/mole.
 5. The ceramic precursor batch composition of claim 1wherein the hydrophobically modified cellulose ether binder has amolecular weight of from about 100,000 g/mole.
 6. The ceramic precursorbatch composition of claim 1 wherein the aqueous solvent is water. 7.The ceramic precursor batch composition of claim 1 wherein MC/W is lessthan about 0.22.
 8. The ceramic precursor batch composition of claim 1wherein the hydrophobically modified cellulose ether binder comprisesmethylcellulose, ethylhydroxy ethylcellulose, hydroxybutylmethylcellulose, hydroxymethylcellulose, hydroxypropyl methylcellulose,hydroxyethyl methylcellulose, hydroxybutylcellulose,hydroxyethylcellulose, hydroxypropylcellulose, sodium carboxymethylcellulose or mixtures thereof.
 9. The ceramic precursor batchcomposition of claim 1 wherein the hydrophobically modified celluloseether binder comprises at least two hydrophobically modified celluloseethers having different molecular weights.
 10. The ceramic precursorbatch composition of claim 1 wherein the hydrophobically modifiedcellulose ether binder comprises at least two hydrophobically modifiedcellulose ethers having different molecular weights and wherein the atleast two hydrophobically modified cellulose ethers have differenthydrophobic groups, different concentrations of the same hydrophobicgroup or both.
 11. The ceramic precursor batch composition of claim 1wherein the ceramic precursor batch composition comprises aluminumtitanate-forming ingredients.
 12. The ceramic precursor batchcomposition of claim 1 wherein ceramic precursor batch compositioncomprises from about 3 wt % to about 10 wt % of the hydrophobicallymodified cellulose ether binder.
 13. The ceramic precursor batchcomposition of claim 1 wherein the composition further comprisescordierite, mullite, clay, talc, zircon, zirconia, spinel, aluminas andtheir precursors, silicas and their precursors, silicates, aluminates,lithium aluminosilicates, alumina silica, feldspar, titania, fusedsilica, nitrides, carbides, borides, silicon carbide, silicon nitride,soda lime, aluminosilicate, borosilicate, soda barium borosilicate ormixtures of thereof.